Several types of communications networks exist today, including, for example, computer networks such as wide area networks (WANs), metropolitan area networks (MANs), local area networks (LANs), and personal area networks (PANs), and cellular networks. User terminals may communicate with the networks wirelessly, e.g., through radio frequency (RF) connections or infrared (IF) connections. The interface between the network and such wireless user terminals is generally called the air interface. Interface devices exist on both sides of the air interface. The interface device in the user terminal may be a wireless adapter, a cellular phone, etc. The interface device in the network may be a base station, an access point, an access network, etc. A network generally contains multiple network interface devices, each communicating with user terminals within a particular area.
The air interface in different networks may use different wireless technologies, may operate on different frequencies, may adopt different communication protocols, and may provide different data rates. A network interface device may provide network access to multiple user terminals using access schemes such as time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), or a combination thereof. The network interface device and the user terminals also follow a set of rules referred to as a communication protocol in communicating with each other. A radio access technology, or RAT, generally refers to the combination of the access scheme and the communication protocol of the air interface of a network.
A radio access technology may be suited better for a particular type of service than for other services. For example, the radio access technology of a cellular network is most efficient in providing cellular services, i.e., roaming telephone services; and the radio access technology of a computer area network is suited for providing fast data connections to components within its limited area. A network with a limited coverage area is generally capable of providing higher data rates or a better connection quality than a network with a more expansive coverage area.
A current trend is to integrate existing networks so that users can enjoy the benefits of all of the existing networks. For example, when a user browses the Internet on a train using a cellular phone, the only accessible network may be a cellular network. However, when the user arrives home or at the office, an area network such as a LAN may be a better network because of the higher data rates the LAN provides. In such a situation, it is desirable to allow the cellular phone to have access to the LAN and also desirable to allow the cellular phone to automatically switch from the cellular network to the LAN without having to first disconnect from the cellular network. Similarly, in a system with multiple network interface devices associated with multiple RATs, it may be desirable to allow a user terminal to switch between the RATs without interrupting the communication. Such a switch between different RATs is often referred to as vertical handover (VHO). In contrast to a vertical handover, horizontal handover takes place in networks with a single RAT.
Before a horizontal or vertical handover takes place, the network interface device in communication with the user terminal, generally referred to as the serving network interface device, has to schedule a time interval during which the communication between the serving network interface device and the user terminal is temporarily suspended, and the user terminal measures the signals of neighboring network interface devices. The measurements are referred to hereinafter as handover measurements. Based on the measurement results, the user terminal or the network interface device or the network determines whether a handover should take place.
Conventional scheduling of handover measurements in a system using a single RAT is explained with reference to FIG. 1, which illustrates the scheduling of handover measurements defined in the IEEE 802.16e standard.
In IEEE 802.16e, the network interface devices are base stations, and the user terminals are mobile stations. Each base station continuously transmits broadcast signals that can be detected by all mobile stations. When a mobile station (MS) needs to measure the broadcast signals from neighboring base stations in preparation of a handover, communication with the serving base station is temporarily suspended. The measurement results may be reported back to the serving base station. Based on the measurement results, the network, the base station, or the user terminal determines whether a handover should take place.
The MS may initiate the handover measurements on its own by sending a request to the serving base station. Alternatively, the serving base station may issue a command to the MS to initiate the measurements. The period from the initiation of the handover measurements to the completion of all the measurements and necessary reports thereof is referred to as a handover measurement period.
In the example shown in FIG. 1, the handover measurements are initiated by the base station. The serving base station is base station BS1, and the neighboring base stations include base stations BS2 and BS3. BS1 issues a command “MOB_SCN-RSP” to the MS to instruct the MS to measure signals from the neighboring base stations including BS2 and BS3. (Step 101.) The MOB_SCN-RSP command includes several parameters: “start frame,” “scanning interval,” “interleaving interval,” and “iteration.” “Start frame” specifies when the MS should start the measurements, “scanning interval” specifies how much time the measurements should take, “iteration” specifies how many scanning intervals are allocated for the measurements, and “interleaving interval” specifies the time interval between two adjacent scanning intervals. Normal communication between the MS and BS1 is temporarily suspended during the scanning interval and resumes during the interleaving interval. If measurement results need to be reported, the report is submitted during the interleaving interval. “Start frame,” “scanning interval,” and “interleaving interval” are all specified in numbers of frames. A frame is a data unit upon which a network operates, and consists of a specified number of bits of information including user data and network overhead information. For a particular RAT, a frame may have a particular time duration. Therefore it is customary to specify the size of a frame in units of time, and to specify time durations in units of frame. In the example shown in FIG. 1, the MS should start the measurement at the M-th frame after the MS receives the MOB_SCN-RSP command, measure the signals from BS2 and BS3 for N frames, resume communication with BS1 for P frames, and measure additional neighboring base stations during the additional scanning intervals.
During the scanning interval, the MS detects the broadcast signals from BS2, synchronizes to BS2, and measures the signals from BS2. (Step 102.) Then the MS repeats the same process for BS3. (Step 103.) If the signals from a neighboring base station are weak, the MS does not need to report the measurement results to BS1. If the signals from a neighboring base station are strong, then the MS will need to submit a report to BS1. A threshold parameter may be set by the network for determining when a report needs to be submitted. FIG. 1 assumes that the signals from at least one of BS2 and BS3 exceed the threshold, and the MS needs to report back to BS1.
At the beginning of the interleaving interval, normal communication between the MS and BS1 resumes, and the MS sends a request to BS1 for additional bandwidth for submitting the report. (Step 104.) Upon receiving allocation of resource (Step 105) for the submission of the report, the MS sends the measurement results to BS1 using the allocated resource. (Step 106.) The report will be used by the network to determine whether the MS should switch over to BS2 or BS3. Alternatively, the MS may determine based on the measurement results if a handover is desired, and include in the report a request for handover.
If multiple scanning intervals have been allocated, the MS enters into another scanning interval at the end of the interleaving interval to measure signals from additional neighboring base stations.
In accordance with the IEEE 802.16e standard, the serving base station and its neighboring base stations all use the same RAT, all share the same frame size, and are all synchronized with one other. Because the time required for handover measurements depends on the timing information of the base stations to be measured, the timing uniformity across a single-RAT network renders determination of scanning intervals simple. Such a timing uniformity, however, does not exist in a multi-RAT system.